A number of cities across the country have decided to build Wi-Fi networks that l hover over residents like a cloud and provide free Internet access.

It's been a lot harder to deliver on those promises than people thought.

First, there have been a lot of difficulties in deploying the networks. How strong should the signal be? Should it be able to work in everyone's home? At what cost? These and other questions have surfaced.

While networks are continuing to be built, the next question is still being fleshed out.

Stefan Weitz, a director of planning at MSN, said they are experimenting to determine which ads work better in the Wi-Fi environment.

"These are ad-supported networks, so they are free, but we all know that nothing is actually free," he said. "So we are working with these folks to find the right mix of advertising and to optimize the usage, so users are not bombarded with ads when they are on the networks."

One Ultramercial Weitz has seen involves Nike, where prior to showing a video, the ad asks whether the viewer is a runner or a basketball player. The ad can then be be tailored to that person. Here's an example on the JiWire Web site.

"They are cool, they are interesting and immersive," he said. "Advertisers love them because they have hugely high click-through rates and they aren't some banner, they are interesting."